Our work is focussed on the molecular mechanisms responsible for establishing and maintaining stable states of gene expression during vertebrate embryogenesis. Progress has been achieved in three key areas: 1. We have defined at single base pair resolution the protein-nucleic acid interactions over active and inactive 5S ribosomal RNA genes in vivo. These results strongly support a role for stable transcription complexes in maintaining gene activity and for nucleosomes in maintaining the repressed state. 2. We have defined the structure of DNA in a nucleosome and the contributions of histone domains to that structure. We find that DNA sequences of widely divergent structure in solution adopt the same structure in the nucleosome. Purified transcription factors interact with comparable affinity to their promoters as naked DNA and when associated with histones H3/H4. Histone H2A/H2B, when incorporated into a nucleosome inhibit transcription factor binding. These results provide a molecular explanation for the access of transcription factors to DNA during staged chromatin assembly at the replication fork. 3. We have defined a new DNA binding motif in the Y-box transcription factors. Germ cell selective variants of these factors have been cloned from Xenopus and mouse respectively suggesting a developmental role that has been conserved through evolution.